Piezoelectric vibration devices, as typically represented by piezoelectric vibrators, are widely employed in mobile telephones and other mobile communication devices. One of the piezoelectric vibration pieces employed in the piezoelectric vibrators is a tuning fork type piezoelectric vibration piece. A tuning fork type piezoelectric vibration piece has a shape of a tuning fork composed of a base portion and a pair of vibrating leg portions (hereinafter called “leg portions”) extending in one direction from the base portion. A tuning fork type piezoelectric vibrator using the tuning fork type piezoelectric vibration piece is widely used as a clock source in a watch.
Recent trends toward miniaturization and improved performance for communication devices necessitate miniaturization and improved characteristics for a tuning fork type piezoelectric vibration piece. In particular, when a tuning fork type piezoelectric vibration piece is made smaller, its leg portions also become narrower and thinner. Unlike a conventional vibration piece which can suppress the influence of higher harmonics, such a small vibration piece allows higher harmonics to oscillate at a low drive level and is more likely to be adversely affected by higher harmonics. For example, in order to suppress a CI increase associated with miniaturization of a vibration piece, a tuning fork type piezoelectric vibration piece which has grooves in principal surfaces of the leg portions needs to set a greater ratio of the groove width to the leg portion width and a greater groove depth than a conventional vibration piece. In this case, banks formed between the ends of the leg portions and the grooves have a narrower width, and the thickness between the grooves in the two principal surfaces is decreased, in comparison with the conventional vibration piece. Eventually, this structure deteriorates the rigidity of the leg portions and tends to allow higher harmonic oscillation. Currently, use of a small-sized tuning fork type piezoelectric vibrator tends to cause oscillations at higher harmonic frequencies or other troubles, depending on the oscillation circuit constant or the oscillation circuit design.
For example, the tuning fork type piezoelectric vibration piece as described in Patent Literature 1 is configured to improve its electrical characteristics in order to reduce the influence of unwanted higher harmonics. According to Patent Literature 1, a tuning fork type piezoelectric vibration piece which has a base portion and a pair of leg portions projecting side by side from the base portion includes an arrangement as described below.
In the tuning fork type piezoelectric vibration piece disclosed in Patent Literature 1, each of the leg portions is composed of a vibrating portion having a drive electrode, and a weight portion formed at a tip end of the vibrating portion and having a greater width than the vibrating portion. Each leg portion is also provided with a groove which extends into the weight portion beyond the connecting position between the vibrating portion and the weight portion. Owing to the wide weight portion at the tip end of the vibrating portion, the tuning fork type piezoelectric vibration piece disclosed in Patent Literature 1 can suppress generation of higher harmonics and can stabilize the vibrational frequency, even though the vibrating portion is made shorter.
Nevertheless, in the tuning fork type piezoelectric vibration piece having such a weight portion, the rigidity of the piezoelectric vibration piece changes sharply at the connecting portion between the vibrating portion and the weight portion. Undesirably, this sharp change in rigidity destabilizes vibrations of the weight portion. To solve this problem, the tuning fork type piezoelectric vibration piece disclosed in Patent Literature 1 provides the groove at the connecting portion between the vibrating portion and the weight portion, thereby alleviating a sharp change in rigidity at the connecting portion between the vibrating portion and the weight portion and ensuring stable vibrations.